Make compatible with Python 3.9 and 3.10 and improve build piple line a bit

[lumbric]

This could be two separate PRs, hope it's not too confusing to mix it in one: first there is some cleanup of the build pipeline stuff, then Python 3.9 and 3.10 is added. In 3.9/3.10 not much has changed, but some things are deprecated (and in 3.10 some Python error messages got way better).

[codecov-commenter]

Codecov Report

Merging #51 (1abf7d0) into master (e751050) will increase coverage by 0.09%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
+ Coverage   85.69%   85.78%   +0.09%     
==========================================
  Files          44       44              
  Lines        3698     3701       +3     
  Branches      835      863      +28     
==========================================
+ Hits         3169     3175       +6     
+ Misses        441      434       -7     
- Partials       88       92       +4     

Impacted Files	Coverage Δ
setup.py	0.00% <ø> (ø)
abcvoting/genprofiles.py	85.56% <100.00%> (ø)
abcvoting/abcrules_cvxpy.py	18.33% <0.00%> (ø)
abcvoting/abcrules_gurobi.py	15.00% <0.00%> (ø)
tests/test_instances/generate.py	9.25% <0.00%> (ø)
abcvoting/abcrules.py	94.28% <0.00%> (+0.08%)	:arrow_up:
abcvoting/scores.py	91.33% <0.00%> (+0.13%)	:arrow_up:
examples/random_profiles.py	85.36% <0.00%> (+0.36%)	:arrow_up:
abcvoting/fileio.py	77.48% <0.00%> (+0.52%)	:arrow_up:
tests/test_fileio.py	100.00% <0.00%> (+1.01%)	:arrow_up:

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